A wet-type multiplate clutch structure is used as a clutch structure in a transmission of a transporting machine. The wet-type multiplate clutch structure comprises a coned disc spring used for absorbing shocks occurring during the engaging of a clutch. FIG. 3 is an enlarged sectional side view showing a composition of a multiplate clutch structure 100 provided with a conventional coned disc spring 200. The multiplate clutch structure 100 comprises a cylindrical clutch drum 101 having a bottom. Plural spline grooves 101A extending in the axial direction are formed on the inner peripheral surface of the clutch drum 101 along the circumferential direction. In the clutch drum 101, a tubular clutch hub 102 is provided thereto and has the common rotation axis with respect to the clutch drum 101. Plural spline grooves 102A extending in the axial direction are formed on the outer peripheral surface of the clutch hub 102 along the circumferential direction.
Driven plates 103 and driving plates 104 are alternately arranged at predetermined spaces between the clutch drum 101 and the clutch hub 102. The driven plate 103 is closely fitted to the spline groove 101A of the clutch drum 101, and the driving plate 104 is closely fitted to the spline groove 102A of the clutch hub 102, and they can move in the axial direction. A piston 105, which is movable in the axial direction, is provided at the bottom side of the clutch drum 101. An oil pressure space 106 is provided between the piston 105 and the clutch drum 101.
A coned disc spring 200 having a hole in the center and a circular dish shape is provided between the driven plate 103, which is on the bottom side of the clutch drum 101, and the piston 105. The coned disc spring 200 is supported by the driven plate 103 at the surface of the outer periphery, and by the piston 105 at the back of the inner periphery.
In the above multiplate clutch structure 100, when operating oil is supplied to the oil pressure space 106, the piston 105 driven by oil pressure presses the driven plate 103, which is on the bottom side of the clutch drum 101, through the coned disc spring 200. Then, the driven plate 103 on the bottom side of the clutch drum 101 moves to the opening of the clutch drum 101, and friction surfaces of the driven plate 103 and the driving plate 104 which are placed oppositely to each other, engage, whereby they are clutched. In this case, the coned disc spring 200 elastically deforms from the dish shape to become flattened, thereby absorbing shocks that occur during the engaging of the clutch.
The above coned disc spring 200 has a circular shape, whereby it tends to relatively rotate with respect to the clutch drum 101 during rotation of the clutch structure 100. Therefore, the coned disc spring 200 frequently hits the inside wall of the clutch drum 101, which may cause a problem of wear on the inside of the wall of the clutch drum 101.
In order to solve the above problem, a coned disc spring 300 has been proposed (for example, see Japanese Unexamined Patent Application Publications Nos. 2001-295860 and H9-329155). As shown in FIGS. 4A to 4C, the coned disc spring 300 comprises a body 310 having a hole 310A in the center and a circular dish shape, and plural teeth 311 are formed on the outer periphery of the body 310. The body 310 of the coned disc spring 300 is arranged at the inside of the clutch drum 101, so that the teeth 311 are closely fitted into the spline grooves 101A on the inner peripheral surface of the clutch drum 101, whereby the coned disc spring 300 does not relatively rotate as described above.
When the above coned disc spring 300 is elastically deformed during the engaging of a clutch, a concentration of stress occurs at the base of the teeth 311. Therefore, there has been a problem in that the durability of the coned disc spring 300 is greatly decreased.
In order to reduce the stress occurred at the base of the teeth 311, a radius of curvature R of a circular arc portion 311A at the base of the teeth 311 may be enlarged. In this case, the circular arc portion 311A at the base of the teeth 311 extends in the circumferential direction and projects to the radial outer side. Therefore, if a corner portion of the spline groove 101A of the clutch drum 101 is sharp, the base of the tooth 311 heavily interfere with the spline groove 101A during rotation of the clutch drum 101. Accordingly, wear of the spline groove 101A caused by the tooth 311 increases. Moreover, a length L of a straight portion 311B on the side of the tooth 311 is shortened, and the area in which the tooth 311 and the spline groove 101A are closely fitted is decreased. Therefore, a contact pressure due to the tooth 311 at the spline groove 101A is increased, and the wear of the spline groove 101A caused by the tooth 311 is increased. Specifically, in order to ensure that the display of the correct mileage of an automobile, which has been increasing recently, the coned disc spring 300 is required to be highly durable. Accordingly, the above problems need to be solved.